1. Field of the Invention
The present invention relates generally to apparatus for tensioning wrapping material, and more particularly, to a pretensioner assembly for stretching a film web prior to wrapping it around a pallet load.
2. Description of the Prior Art
In the material handling art, it is common practice to load boxes or sacks of material onto a pallet for easy transportation and storage. A pallet is a tray or platform which is used in conjunction with a forklift for lifting and moving materials. Such loading arrangements in which a large number of sacks or boxes are stacked upon a wooden pallet are easily upset when moved and, therefore, must be stabilized in some manner to prevent the stack from falling over. A common method for stabilizing a palletized load is to spiral or full wrap the load with a high strength material such as stretchable plastic film or netting. This method is referred to as "unitizing".
According to conventional practice, it is known to provide a spiral wrap apparatus for palletized loads, in which a rotatable platform is situated horizontally for receiving the palletized load, and a stretchable film web is dispensed from a vertical mandrel situated adjacent the platform. According to this method, the width of the stretchable film is less than the height of the load to be wrapped, and the mandrel supporting the spool of film is located on a carriage assembly which is raised and lowered in the vertical direction while the wrapping operation is carried out. As the palletized load rotates on the platform, the spool of film is moved vertically and then downwardly, wrapping the load in a helical pattern and thereby unitizing the load.
It is also known to provide wrapping apparatus in which a single sheet of stretchable film material is wrapped about a palletized load as the load is rotated.
In the foregoing pallet wrapping methods, the stretchable film web is dispensed from a feedstock roll having at least several hundred linear feet of wrapping material. The film web is pulled from the roll by the pallet load as it rotates. The film may have wrinkles or folds which should be smoothed out prior to wrapping. It is also desirable that the film web be applied under tension so that it fits tightly around the loaded pallet. Moreover, since the cost of the film web material is a significant part of the overall handling cost, it is desirable to pre-stretch the film material as much as possible.
Several systems have been designed to supply stretchable wrapping material under tension to a pallet load. These methods include applying a brake to the mandrel on which the feedstock roll is mounted, or applying pressure to the wrapping material as it is unrolled by using a separate roller pressed against the feedstock roll. In these and other conventional methods, the wrapping material is pulled off of the feedstock roll by the rotation of the pallet load. The feedstock roll is braked so that the linear speed of the web wrapping material, as it leaves the feedstock roll, is slightly less than the linear speed of the web as it is pulled around the pallet. This provides for stretching of the wrapping material between the feedstock roll and pallet load. The stretching action pulls the wrinkles out of the web and puts it under tension so that it will fit snugly around the pallet load.
One limitation of the foregoing dispensing method is that the tension on the wrapping material is not constant. Pallet loads are usually irregular in shape and rotate in an eccentric manner due to inexact centering on a turntable. The pallet load and the roll of film interact in a manner similar to a pulley system in response to turntable rotation. The pallet load size, shape and placement on the turntable affects the velocity of the inner pulley (the roll of film). As a result, the film web is wrapped around the loaded pallet at speeds which vary throughout the wrapping operation. Since the wrapping material is pulled from the feedstock roll by the action of the rotating pallet load, these irregularities cause the tension on the wrapping material to vary. The pallet load, when wrapped in this manner, usually has tight zones and loose zones, which may cause the load to be unstable.
Because wrapping material is pulled off the feedstock roll directly by the pallet load, substantial forces are induced in the web while unrolling the wrapping material and stretching it to create tension. The pallet load may consist of relatively light objects which are easily displaced by the uneven pulling forces. In an attempt to overcome this limitation, some pallet wrapping devices include a movable member, commonly referred to as a hold-down foot, which is engagable with the top of the pallet load. The foot holds the load in place while the wrapping material is being applied. However, very light loads can still be upset by the pressure exerted by the wrapping material, and the hold-down foot may not be compatible with fragile or irregularly shaped loads.